For high-efficiency switching power converter, because the whole load current will flow through a switch when switching, the switch has to bear a very high switching stress and a high switching loss proportional to the switching frequency, zero voltage switching is a very important technique.
Besides, time-varying current and time-varying voltage caused by high-speed switching will result in serious electromagnetic interference (EMI).
In order to reduce the size and weight of a converter and enhance its power density, it is necessary to increase its switching frequency, which will more deteriorate the drawbacks of the switching converter. If the voltage of the switch can be made zero at the instant of switching, the above drawbacks can be improved. Therefore, how to reduce the switching loss, the switching stress and EMI of a switching converter is a very important problem to be urgently solved in this industry.
FIG. 1A shows a conventional voltage-drop type resonance converter having zero voltage switching. This kind of converter makes use of a resonance capacitor in shunt with a switch to produce a zero voltage so that zero voltage switching can be accomplished for the switch.
Although the above zero voltage switching technique can reduce the switching loss generated when switching due to zero voltage switching, the resonance circuit will cause a large number of tank currents, which will cause transmission loss in the circuit. This transmission loss will seriously affect the supply and transmission of power to lower the conversion efficiency of the power source.
Accordingly, the present invention aims to provide a switching power conversion circuit, which not only can accomplish zero voltage switching with a simple circuit to lower the switching loss, but also can diminish the generation of tank current to greatly lower the transmission loss.